Capacitive pressure sensors are used in demanding applications such as industrial transmitters and aerospace probes. Sensor bodies are formed from stacked layers of low hysteresis dielectric material such as sapphire, silicon or ceramic. At least one layer in the stack includes a thinned diaphragm region that is deflected by the pressure. A metal capacitor plate is deposited on the diaphragm region and an opposite support plate to form a capacitor. The metal capacitor plate on the diaphragm can cause problems because the metal creeps when the diaphragm deflects, leading to hysteresis errors in the measured pressure Deposition of the capacitor plate on the diaphragm and a lead to the capacitor plate are manufacturing processes that can be costly to implement and control in mass production.
The problem with hysteresis errors due to the presence of metal on the deflecting low hysteresis diaphragm material becomes increasingly important as other sources of pressure sensor error are corrected through use of improved diaphragm materials, improved bonding such as direct bonding and improved stress isolation in mounting sensors and electrical leads. A technology is needed that avoids the problems with depositing metal on diaphragms and the creep or hysteresis in metals on deflecting diaphragms in pressure sensors in demanding applications.
A pressure sensor includes a diaphragm that has a dielectric portion that moves in a cavity near capacitor plates that are fixed relative to a mounting frame.
The diaphragm is supported on the frame and the frame surrounds the cavity. The diaphragm has an outer surface that receives pressure and has an inner surface facing the cavity. The inner surface carries a dielectric portion that is movable relative to the frame by the pressure.
The capacitor plates are not on the deflecting diaphragm, but are both fixed. The capacitor plates sense movement of the nearby dielectric portion of the deflecting diaphragm and generate an electrical output representative of pressure.